Question: Why do Islamic prayer times change from day to day?
Muslims pray at five appointed times each day. The precise timing of these prayers often vary by a few minutes each day, so that over the course of a year there is a wide range of prayer times. Some communities publish prayer "schedules" to keep track of the times, and specialized computer programs can also calculate the prayer times for each area, for each day of the year. Why is this so? Why don't Muslims just pick the times and stick with them year-round?
Answer: The Islamic prayers are fixed at the same "time" period each day -- before dawn, noon, afternoon, sunset, and evening. Because of the rotation of the earth, the revolution of the earth around the sun, the tilt of the earth, the various latitudes of the earth's locations, daylight savings time, etc. -- the "times" (according to the clock) for these prayers do change from day to day and depend on location. Islamic prayer times were traditionally set according to the movement of the sun, not of the clock, and this is how they continue to be observed.